I. Field of the Invention
This invention relates generally to a device and method for pacing a patient""s heart and more particularly to a device and method for improving the hemodynamic performance of patients suffering from heart failure through multi chamber pacing. The patient in need of improving hemodynamic performance may suffer from, for example, congestive heart failure (CHF), or other heart failure requiring pacing even though intrinsic PR conduction is present. The device includes a means for tracking an R-wave associated with intrinsic conduction in a ventricle of the patient""s heart. The method includes tracking a sensed R-wave for a predetermined RV delay interval and then stimulating the ventricle if a P-wave is sensed during the preset simultaneous Post Ventricular Atrial Refractory Period (PVARP) interval.
II. Discussion of the Related Art
Typically, a patient suffering from a higher degree of AV-block or an AV conduction disorder is implanted with a conventional atrial tracking (DDD or VDD) pacemaker suited for pacing the ventricle. Such a pacemaker is designed to pace the ventricle after a pre-set AV delay, synchronous with the intrinsic atrial rate. The purpose of these pacemakers is to make sure that heart beats are properly timed and not omitted. Intrinsic rhythm is favorable over paced rhythm for both hemodynamic and economic (battery conservation) reasons. It is also important and necessary to prevent pacing the ventricle during the vulnerable period for inducing arrhythmia immediately following an intrinsic ventricular beat. Hence, ventricular pacing is inhibited when intrinsic conduction from the SA node causes an R-wave to be sensed prior to the scheduled ventricular pace.
Patients suffering from congestive heart failure (CHF), for example, either do not exhibit heart block at all or often only suffer from a first degree AV-block or a slightly prolonged delay interval between the depolarization of the atrium and the depolarization of the ventricle. Recent research has shown that acute hemodynamic performance, exercise tolerance and quality of life of CHF patients can be improved by a pre-excitation of the ventricles with pacing following normal sinus beats. Patients benefiting from pre-excitation of the ventricles experience a return of heart failure symptoms immediately upon omission of pre-excitation pacing. Thus, for these patients, it is important that the paced pre-excitation of the ventricle be performed continuously in order to improve the contraction pattern, even though intrinsic beats would occur slightly later if there were no pacing. When pacing a patient suffering from CHF, it is highly undesirable to omit pacing when it is supposed to occur.
When pacing the heart of a CHF patient having normal intrinsic (PR) conduction, although continuous pacing is desirable, use of a conventional atrial tracking dual chamber pacemaker completely inhibits ventricular pacing when the intrinsic atrial rate (hereinafter the sinus rate) rises above a preprogrammed atrial maximum tracking rate (AMTR). These pacemakers also exhibit an undesirable pronounced pacing hysteresis, wherein ventricular pacing is not resumed until the sinus rate falls below a ventricular inhibition threshold rate (VIR). Also, in these pacemakers, a premature ventricular contraction (PVC) may inhibit ventricular pacing when the sinus rate exceeds the VIR.
It would be advantageous to eliminate the pacing hysteresis, while extending the limit or MTR for pacing of the ventricle. However, this is not possible with the conventional atrial tracking multi-chamber pacemaker. A CHF patient, for example, has an elevated sinus rate in order to maintain normal cardiac output despite reduced cardiac pumping efficiency and, therefore, requires a high atrial MTR. Together with an elevated sinus rate, many such patients also have prolonged PR intervals and correspondingly prolonged retrograde conduction intervals requiring long PVARP intervals. The required long PVARP intervals prevent tracking of retrograde P-waves, thereby reducing pacemaker mediated tachycardia (PMT). In conventional atrial tracking pacemakers, the highest allowable atrial MTR is determined in part by the length of the PVARP interval, which may limit the atrial MTR to a rate that is below the normal range of sinus rates in the CHF patient. Hence, use of a conventional atrial tracking dual chamber pacemaker would not allow continuous ventricular pacing above the atrial MTR. Therefore, there is a need for a dual chamber pacemaker and a method of operating the same that may be used to pace the failing heart of a patient having intrinsic conduction, wherein the pacemaker provides for continuous pacing of the ventricle at a sinus rate that exceeds an atrial maximum tracking rate and does not exhibit pacing hysteresis below the MTR. The present invention addresses this need.
The purpose of the present invention is to provide a device and method of pacing continuously, without hysteresis, the ventricles of a patient""s failing heart having intrinsic conduction even when the sinus rate rises above a preset atrial maximum tracking rate.
Conventional dual chamber pacemakers commonly have a combination of dual chamber sensing, atrial sensing, ventricular sensing, dual chamber pacing, atrial pacing, ventricular pacing, and atrial tracking. A conventional dual chamber pacemaker may be modified according to the present invention to include a ventricular tracking mode and thereby increase the range of pacing therapy. When used with a patient having intrinsic (PR) conduction, the ventricular tracking mode minimizes the loss of ventricular pacing output as the sinus rate rises above a preset atrial MTR. As the atrial MTR is exceeded by the sinus rate, the ventricular tracking pacemaker restores a Wenckebach-like pacing behavior, thereby continuing ventricular pacing. Those skilled in the art will appreciate that this pacing, although similar to Wenckebach pacing behavior is not Wenckebach pacing.
During this Wenckebach-like pacing, the ventricular tracking pacemaker paces the ventricle due to atrial tracking unless a legitimate P-wave is sensed during a preset post ventricular atrial refractory period (PVARP). A legitimate P-wave refers to a signal or wave that corresponds to an intrinsic atrial depolarization. When a legitimate P-wave is sensed during the PVARP, the ventricular tracking pacemaker tracks a preceding sensed R-wave (due to intrinsic PR conduction) and paces the ventricle after a preset delay interval, hereinafter referred to as the RV delay. Alternatively the ventricular pace can occur after a preset delay from the P-wave sensed during PVARP, hereinafter referred to as the refractory atrial to ventricular (RAV) delay. As the sinus rate continues to increase, the sinus rate reaches a 2:1 ventricular tracking rate up to a ventricular MTR or limit at which point ventricular pacing is inhibited. The conventional dual chamber pacemaker ignores legitimate P-waves sensed during PVARP and does not pace the ventricle above the atrial MTR. With the ventricular tracking pacemaker, as the sinus rate decreases from the ventricular MTR, there is no significant pacing hysteresis commonly found in the conventional atrial tracking pacemakers.
The ventricular tracking pacemaker includes a means for sensing an atrial event and transmitting a signal associated with the sensed atrial event, means for sensing a ventricular event and transmitting a signal associated with the sensed ventricular event, means for tracking a P-wave, means for tracking an R-wave, means for selectively stimulating a preselected ventricle of the patient""s heart, and a means for controlling the selective stimulation of the ventricle. The means for controlling the stimulation is electrically coupled to the sensing, tracking and stimulating means. The means for controlling the stimulation controls the stimulation to the preselected ventricle in accordance with a timing sequence which is dependent upon the tracked ventricular and tracked atrial events.
In the preferred embodiment, the means for controlling determines the intrinsic ventricular rate from the signal corresponding to sensed ventricular events. If the intrinsic ventricular rate is greater than the preset ventricular maximum tracking rate, the means for controlling inhibits the stimulation to the ventricle.
It is accordingly a principal object of the present invention to provide a multi chamber pacemaker for pacing the selected ventricles of a patient suffering from heart failure but having intrinsic PR conduction, wherein the ventricle may be paced at a rate that is above an atrial maximum tracking rate.
A further object of the present invention is to provide a multi chamber pacemaker for pacing the selected ventricles of a patient, wherein the selected ventricles are paced a preset time after an R-wave is tracked by the pacemaker.
Another object of the present invention is to provide a method of multi chamber pacing that paces the selected ventricles a preset time after detecting an intrinsic conduction transmitted from the ventricles.
Yet another object of the present invention is to provide a method of multi chamber pacing which paces the ventricles a preset time after a P-wave is sensed during a PVARP interval following a sensed R-wave, wherein pacemaker-mediated tachycardia is prevented if the P-wave is due to retrograde conduction.
Still another object of the present invention is to provide a pacemaker that may pace the ventricle above the atrial MTR, wherein there is no significant pacing hysteresis when the atrial rate returns to a lower rate limit.
These and other objects, as well as these and other features and advantages of the present invention will become readily apparent to those skilled in the art from a review of the following detailed description of the preferred embodiment in conjunction with the accompanying drawings and claims and in which like numerals in the several views refer to corresponding parts.